『Abstract
Grus is an ill-defined product of deep weathering of coarse-grained
rocks whose relationships to other weathering changes remain unclear.
This paper attempts to address this issue by reviewing a number
of examples of coarse saprolites from a variety of climatic and
topographic settings. Grus is the category of weathering mantle
that possesses the following characteristics: sand+gravel 75-100％;
silt+clay ＜25％; clay ＜10％; Chemical Index of Alteration (CIA)
60-70; Chemical Weathering Index (CWI) 15-20. The origin of grus
is connected with weakening of rock fabric by development of microcracks,
biotite expansion, and initial alteration of plagioclase. It may
originate either beneath the surface or at greater depths within
a weathering profile. The climatic approach to the formation of
grus mantles offers limited explanation of field occurrences,
as these materials are widespread across climatic zones, from
the humid tropics to cool temperate areas, although rates of grusification
are likely to be influenced by climatic parameters. By contrast,
topographic and also petrographic factors appear to play key roles
in the development of grus, which may be regarded as a response
of weathering systems to rapid relief differentiation. Grus mantles
are preferentially associated with moderate to high relief; hence,
they are essentially azonal and their development is under way
in many areas of the world.
Keywords: Arenes（最初のeの頭に｀）; Chemical weathering;
Granite weathering; Grus; Saprock; Weathering profiles』
1. Introduction
2. Grus as a weathering product
2.1. The nature of ‘grus’ and its definition
2.2. Surface grus and transitional zones of weathering profiles
3. Petrographic factors in grus weathering
3.1. Microcrack systems and possible hydrothermal alteration
3.2. Influence of rock fabric and mineralogy
4. Distribution of grus in relation to present-day climatic zones
4.1. Grus mantles and present-day climates
4.2. Climatic inheritance
5. Geomorphological context of grus mantles
5.1. The morphotectonic setting of grus mantles
5.2. Grus and relief-interpretation
6. Conclusions
References